RESIDUAL STRAIN MEASUREMENT OF PLAIN WEAVE COMPOSITES USING THE CURE REFERENCE METHOD By NANCY M. STRICKLAND A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2007 © 2007 Nancy M. Strickland 2 In memory of my mother. Also to my family, since none of this would have been possible without their encouragement and support. 3 ACKNOWLEDGMENTS I would like to thank everyone who provided support and assistance throughout this research. First, I want to thank Dr. Peter Ifju for all of the advice and support he has offered. I would also like to thank my other committee members, Dr. Bhavani Sankar and Dr. Raphael Haftka, for their advice and expertise. Also, this work would have not been possible without the help and shared knowledge from Weiqi Yin, Tzu-Chau Chen and Diane Villanueva, my colleagues in the Experimental Stress Analysis Lab. Finally, I would like to thank my mom, dad and the rest of my family for all of their support and encouragement they have provided throughout the years. 4 TABLE OF CONTENTS page ACKNOWLEDGMENTS ...............................................................................................................4 LIST OF TABLES ...........................................................................................................................7 LIST OF FIGURES .........................................................................................................................8 ABSTRACT ...................................................................................................................................11 CHAPTER 1 INTRODUCTION ..................................................................................................................13 Woven Composites .................................................................................................................13 Residual Stresses and Strains ..................................................................................................14 Research Objectives ................................................................................................................15 2 LITERATURE REVIEW .......................................................................................................17 Introduction .............................................................................................................................17 Destructive Techniques ..........................................................................................................17 Hole Drilling Method ......................................................................................................17 First Ply Failure Test .......................................................................................................18 Non-Destructive Techniques ..................................................................................................18 X-Ray Diffraction ............................................................................................................18 Embedded Sensors ...........................................................................................................19 Warpage Measurements ..................................................................................................20 Cure Reference Method ...................................................................................................20 Analysis Methods Used in Predicting Residual Stresses in Woven Composites ...................20 3 CRM DEVELOPMENT .........................................................................................................22 Principles of the Cure Reference Method ...............................................................................22 Moiré Interferometry ..............................................................................................................23 Specimen Preparation .............................................................................................................24 Development Process ..............................................................................................................25 Original Cure Reference Method ....................................................................................25 Original cure reference method procedure ...............................................................25 Results using original CRM .....................................................................................26 Silicone Rubber Based Methods .....................................................................................28 Silicone rubber grating using resin from prepreg .....................................................28 Results using silicone rubber grating using resin from prepreg ...............................29 Modifications attempted: Thin silicone rubber grating ...........................................29 Modification attempted: Additional epoxy cured onto silicone rubber ..................30 Results from silicone rubber and 3501-6 .................................................................31 5 Kapton® Polyimide Film Based Methods .......................................................................31 Results using the Kapton® film method ..................................................................32 Modifications attempted to the Kapton® method ....................................................33 High Temperature Epoxy Based Methods ......................................................................33 Pooling the epoxy onto the grating ..........................................................................35 Spreading the epoxy onto the grating .......................................................................35 Spreading the epoxy onto the prepreg before immediately applying to grating ......36 Spreading the epoxy and letting it set before placing on autoclave tool ..................37 Conclusions .............................................................................................................................37 4 RESIDUAL STRAIN MEASUREMENTS ...........................................................................51 Phase Shifting .........................................................................................................................51 Results .....................................................................................................................................52 Conclusion ..............................................................................................................................53 5 FEM SIMULATION OF PROCESS INDUCED STRAINS .................................................59 Motivation for Performing FEM Analysis .............................................................................59 Model Description ..................................................................................................................59 Simulating the Curing Cycle ..................................................................................................60 Using FEA to Validate the Experimental Method ..................................................................61 Experimental Process Induced Residual Stresses ...................................................................62 Conclusions .............................................................................................................................63 6 CONCLUSIONS AND FUTURE WORK .............................................................................73 Conclusions .............................................................................................................................73 Future Work ............................................................................................................................74 APPENDIX CRM IMAGES AND PHASE SHIFTED RESULTS ............................................................76 LIST OF REFERENCES ...............................................................................................................80 BIOGRAPHICAL SKETCH .........................................................................................................83 6 LIST OF TABLES Table page 5-1 Material properties used in ABAQUS model for the plain weave composite RVE ..........65 7 LIST OF FIGURES Figure page 1-1 Representative volume element of the plain weave composite geometry .........................16 3-1 Schematic of a typical four-beam interferometer ..............................................................39 3-2 Fixture used to align the diffraction grating with the autoclave tool. ................................39 3-3 Oven and vacuum lines used for the curing cycle .............................................................40 3-4 Vacuum bagging assembly ................................................................................................40 3-5 Cure cycle used for HMF plain weave prepreg .................................................................41 3-6 Tool used to separate master grating from the autoclave tool ...........................................41 3-7 Tools required to make Teflon device ...............................................................................42 3-8 Specimens made using Original CRM procedure. ...........................................................42 3-9 Specimen made using original CRM with successful transfer. .......................................43